A flywheel (mass damper) is typically connected to a crankshaft of an internal combustion engine in order to reduce rotary fluctuation at low speed. The greater the inertial mass of the flywheel, the smaller the rotary fluctuation, and therefore if the inertial mass of the flywheel is increased, the rotation speed during idling can be reduced, thus improving fuel consumption and emissions performance. However, when the inertial mass of the flywheel is increased, the increase in rotation speed of the engine during vehicle acceleration becomes slow, and furthermore, the effectiveness of the engine brake deteriorates when the vehicle is coasting.
JP-A-S56-70147 published by the Japanese Patent Office in 1981 discloses technology for combining the requirements at low speed with the requirements during acceleration and during coasting by providing a second flywheel that is capable of rotation relative to a crankshaft separately from a first flywheel which is connected to the crankshaft, and by connecting this second flywheel to the first flywheel at times of low speed.